Developments in molecular biology have led to an understanding of how certain therapeutic compounds interact with molecular targets and lead to a modified physiological condition. Specificity of therapeutic compounds for their targets is derived in part from interactions between complementary structural elements in the target molecule and the therapeutic compound. A greater variety of target structural elements in the target leads to the possibility of unique and specific target/compound interactions. Because polypeptides are structurally diverse, researchers have focused on this class of targets for the design of specific therapeutic molecules.
In addition to therapeutic compounds that target polypeptides, researchers also have identified compounds that target DNA. Some of these compounds are effective anticancer agents and have led to significant increases in the survival of cancer patients. Unfortunately, however, these DNA targeting compounds do not act specifically on cancer cells and therefore are extremely toxic. Their unspecific action may be due to the fact that DNA often requires the uniformity of Watson-Crick duplex structures for compactly storing information within the human genome. This uniformity of DNA structure does not offer a structurally diverse population of DNA molecules that can be specifically targeted.
Nevertheless, there are some exceptions to this structural uniformity, as certain DNA sequences can form unique secondary structures. For example, intermittent runs of guanines can form G-quadruplex structures, and complementary runs of cytosines can form i-motif structures. Formation of G-quadruplex and i-motif structures occurs when a particular region of duplex DNA transitions from Watson-Crick base pairing to intermolecular and intramolecular single-stranded structures.